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12.4.6.3. FLUSH Syntax

FLUSH [NO_WRITE_TO_BINLOG | LOCAL]
    flush_option [, flush_option] ...

The FLUSH statement clears or reloads various internal caches used by MySQL. One variant acquires a lock. To execute FLUSH, you must have the RELOAD privilege.

By default, FLUSH statements are written to the binary log so that they will be replicated to replication slaves. Logging can be suppressed with the optional NO_WRITE_TO_BINLOG keyword or its alias LOCAL.

Note

FLUSH LOGS, FLUSH MASTER, FLUSH SLAVE, and FLUSH TABLES WITH READ LOCK are not written to the binary log in any case because they would cause problems if replicated to a slave.

The RESET statement is similar to FLUSH. See Section 12.4.6.6, “RESET Syntax”, for information about using the RESET statement with replication.

flush_option can be any of the following items:

  • DES_KEY_FILE

    Reloads the DES keys from the file that was specified with the --des-key-file option at server startup time.

  • HOSTS

    Empties the host cache tables. You should flush the host tables if some of your hosts change IP number or if you get the error message Host 'host_name' is blocked. When more than max_connect_errors errors occur successively for a given host while connecting to the MySQL server, MySQL assumes that something is wrong and blocks the host from further connection requests. Flushing the host tables enables further connection attempts from the host. See Section B.5.2.6, “Host 'host_name' is blocked”. You can start mysqld with --max_connect_errors=999999999 to avoid this error message.

  • LOGS

    Closes and reopens all log files. If binary logging is enabled, the sequence number of the binary log file is incremented by one relative to the previous file. On Unix, this is the same thing as sending a SIGHUP signal to the mysqld server (except on some Mac OS X 10.3 versions where mysqld ignores SIGHUP and SIGQUIT).

    If the server is writing error output to a named file (for example, if it was started with the --log-error option), FLUSH LOGS causes it to rename the current error log file with a suffix of -old and create a new empty log file. No renaming occurs if the server is not writing to a named file (for example, if it is writing errors to the console).

  • MASTER

    Deletes all binary logs, resets the binary log index file and creates a new binary log. FLUSH MASTER is deprecated in favor of RESET MASTER, and is supported for backward compatibility only. See Section 12.5.1.2, “RESET MASTER Syntax”.

  • PRIVILEGES

    Reloads the privileges from the grant tables in the mysql database. On Unix, this also occurs if the server receives a SIGHUP signal.

    The server caches information in memory as a result of GRANT, CREATE USER, CREATE SERVER, and INSTALL PLUGIN statements. This memory is not released by the corresponding REVOKE, DROP USER, DROP SERVER, and UNINSTALL PLUGIN statements, so for a server that executes many instances of the statements that cause caching, there will be an increase in memory use. This cached memory can be freed with FLUSH PRIVILEGES.

  • QUERY CACHE

    Defragment the query cache to better utilize its memory. FLUSH QUERY CACHE does not remove any queries from the cache, unlike FLUSH TABLES or RESET QUERY CACHE.

  • SLAVE

    Resets all replication slave parameters, including relay log files and replication position in the master's binary logs. FLUSH SLAVE is deprecated in favor of RESET SLAVE, and is supported for backward compatibility only. See Section 12.5.2.5, “RESET SLAVE Syntax”.

  • STATUS

    This option adds the current thread's session status variable values to the global values and resets the session values to zero. It also resets the counters for key caches (default and named) to zero and sets Max_used_connections to the current number of open connections. This is something you should use only when debugging a query. See Section 1.7, “How to Report Bugs or Problems”.

  • TABLES

    FLUSH TABLES has several variant forms. FLUSH TABLE is a synonym for FLUSH TABLES, except that TABLE does not work with the WITH READ LOCK variant.

    • FLUSH TABLES

      Closes all open tables, forces all tables in use to be closed, and flushes the query cache. FLUSH TABLES also removes all query results from the query cache, like the RESET QUERY CACHE statement.

    • FLUSH TABLES tbl_name [, tbl_name] ...

      With a list of one or more comma-separated table names, this is like FLUSH TABLES with no names except that the server flushes only the named tables. No error occurs if a named table does not exist.

    • FLUSH TABLES WITH READ LOCK

      Closes all open tables and locks all tables for all databases with a global read lock until you explicitly release the lock by executing UNLOCK TABLES. This is a very convenient way to get backups if you have a file system such as Veritas or ZFS that can take snapshots in time.

      FLUSH TABLES WITH READ LOCK acquires a global read lock and not table locks, so it is not subject to the same behavior as LOCK TABLES and UNLOCK TABLES with respect to table locking and implicit commits:

      FLUSH TABLES WITH READ LOCK does not prevent the server from inserting rows into the log tables (see Section 5.2.1, “Selecting General Query and Slow Query Log Output Destinations”).

  • USER_RESOURCES

    Resets all per-hour user resources to zero. This enables clients that have reached their hourly connection, query, or update limits to resume activity immediately. FLUSH USER_RESOURCES does not apply to the limit on maximum simultaneous connections. See Section 5.5.4, “Setting Account Resource Limits”.

The mysqladmin utility provides a command-line interface to some flush operations, via command such as flush-hosts, flush-logs, flush-privileges, flush-status, and flush-tables.

Note

It is not possible in MySQL 5.1 to issue FLUSH statements within stored functions or triggers. However, you may use FLUSH in stored procedures, so long as these are not called from stored functions or triggers. See Section D.1, “Restrictions on Stored Routines, Triggers, and Events”.